Auto A/C Terms - Glossary
British Thermal Unit - BTU

Understand that heat can be measured in a couple of different ways. If we use a thermometer, we are measuring the intensity of the heat. That is telling us how hot it is. On the other hand, if you want to know the quantity of heat, you would measure that with BTUs.

When we use BTUs to measure heat, we are determining the AMOUNT or QUANTITY of heat. As an example, a BTU is the quantity of heat that is required to raise the temperature of one pound of water one degree Fahrenheit.

Understanding the measurement of BTUs is important to understanding the principles behind how the A/C system really works and how and when the most efficient heat transfer occurs. Using the example above, if it takes 1 BTU to raise one pound of water 1 degree Fahrenheit, then it would take 180 BTUs to raise that same pound of water from freezing at 32 degrees Fahrenheit to boiling at 212 degrees Fahrenheit.

However, what is critical is understanding how many BTUs are required to cause that one pound of water at 212 degrees Fahrenheit to change state to one pound of vapor. That requires about 5 times the amount of heat.

Therefore the largest amount of heat is transferred when a liquid substance changes state to become a vapor. In auto A/C systems understand that the refrigerant is constantly changing state as the system operates. In the evaporator, the liquid refrigerant is actually boiling and changing state to vapor refrigerant. This is where the largest amount of heat is being transferred. In fact, this is where the largest amount of BTU’s are being removed from the hot air passing over the evaporator.